Field of the Invention
The present invention relates to a capacitive electromechanical conversion device or transducer that performs transmission and reception of acoustic waves, such as ultrasonic waves, and a sample information acquisition apparatus using the capacitive electromechanical conversion device or transducer. The transmission and reception in this specification means at least one of transmission and reception. Although the acoustic waves are used as a term including sound waves, ultrasonic waves, and photoacoustic waves, the acoustic waves may be typified by the ultrasonic waves.
Description of the Related Art
Capacitive Micromachined Ultrasonic Transducers (CMUTs) have been proposed as transducers that performs transmission and reception of the ultrasonic waves (refer to A. S. Ergun, Y. Huang, X. Zhuang, O. Oralkan, G. G. Yarahoglu, and B. T. Khuri-Yakub, “Capacitive micromachined ultrasonic transducers: fabrication technology,” Ultrasonics, Ferroelectrics and Frequency Control, IEEE Transactions on, vol. 52, no. 12, pp. 2242-2258, December 2005). The CMUTs are manufactured using a Micro Electro Mechanical Systems (MEMS) process that applies a semiconductor process. FIG. 13 is a schematic cross-sectional view of an exemplary CMUT (transmission-reception element). Referring to FIG. 13, a set of a vibrating membrane 101, and a first electrode 102 and a second electrode 103, which are opposed to each other with a cavity 105 sandwiched therebetween, is referred to as a cell. The vibrating membrane 101 is supported by supporters 104 formed on a chip 100. A direct-current voltage generating unit 202 is connected to the second electrode 103. Certain direct-current voltage Va is applied from the direct-current voltage generating unit 202 to the second electrode 103 via a second conductive line 302. The first electrode 102 is connected to a transmission-reception circuit 201 via a first conductive line 301 and has fixed potential near ground (GND) potential. This causes a potential difference of Vbias=Va−0V between the first electrode 102 and the second electrode 103. Adjusting the value of the direct-current voltage Va causes the value of Vbias to coincide with a desired potential difference (around several ten volts to several hundred volts) determined on the basis of mechanical characteristics of the CMUT cells.
Application of alternating-current drive voltage to the first electrode 102 from the transmission-reception circuit 201 causes alternating-current electrostatic attractive force between the first electrode 102 and the second electrode 103 and causes the vibrating membrane 101 to vibrate at a certain frequency to transmit the ultrasonic waves. The vibration of the vibrating membrane 101 in response to the ultrasonic waves causes weak current in the first electrode 102 through electrostatic induction. Measurement of the value of the current with the transmission-reception circuit 201 allows a reception signal to be extracted. The potential difference between the CMUT electrodes causes the electrostatic attractive force between the electrodes to decrease the distance between the electrodes. Increase in electric field strength between the electrodes increases transmission sound pressure (transmission efficiency) when the same drive voltage is applied and increases an output signal (reception sensitivity) when the same ultrasonic waves are received.